Starch ester

ABSTRACT

The present invention relates to a starch ester having short- and long-chain acyl groups wherein hydrogens in reactive hydroxyl groups in the same starch molecule have been replaced by C2-4 short-chain acyl groups and C6-18 long-chain acyl groups. The degree of substitution by the short- and long-chain acyl groups are regulated so as to make the starch ester thermo-plasticized and moldable even in the absence of a plasticizer. Accordingly, the starch ester of the present invention can be used as a biodegradable thermoplastic plastic material which can be thermo-plasticized in the absence of a plasticizer.

The present application is a divisional of the patent application Ser.No. 09/647,931 filed Oct. 16, 2000 now U.S. Pat. No. 6,495,679.

TECHNICAL FIELD

The present invention relates to a starch ester wherein reactivehydroxyl groups in the same starch molecule have been replaced by acylgroups and groups derived therefrom (hereinafter referred tocollectively as “acyl groups”). Further, the present invention relatesto a starch ester preferable as a base polymer in biodegradablestarch-based plastic processed articles.

As used herein, the plastic processed articles include molded processedarticles and modified processed articles molding-processed or modifiedas a whole or partially with a plastic composition, and the moldingprocessing includes injection molding, extrusion molding, inflation,T-die extrusion, calendering, compression molding (press molding),transfer molding, casting, laminating, vacuum molding, blow molding,foam molding, coating, flow casting, heat fusion, stretching etc. (see5th Handbook of Chemistry, Applied Chemistry I, p. 773, Table 10.22,Mar. 15, 1995, compiled by the Japanese Society of Chemistry andpublished by Maruzen). Accordingly, the molded processed articlesinclude not only molded articles having a three-dimensional form butalso films, sheets, coated papers, etc. Further, the modified processedarticles include not only papers, processed papers and non-wovenfabrics, but also articles produced by adding starch-substitutedderivatives as modifiers to papers, non-woven fabrics, etc., made ofnatural materials.

BACKGROUND ART

The basic method of modifying starch, associated with the presentinvention, is esterification (acylation), and the starch ester producedby this reaction has been known as low-substituted starch (starch ester)esterified in anaqueous reaction system (“Starch Science Handbook”, K.K. Asakura Shoten, p. 550).

With respect to high-substituted starch ester (esterified starch), amethod of reacting an acid anhydride in pyridine by use of dimethylamino pyridine or an alkali metal as a catalyst (“Starch Chemistry &Technology” authored by Whistler, published by Academic Press, pp.332-336), a method of reacting an acid anhydride at a high temperatureof 100° C. or more by use of an aqueous solution of an alkali metalhydroxide as a catalyst (Japanese National Publication No. 508,185/1993,and p. 73 in the March issue of Die Starke, 1972), and a method ofreaction in a non-aqueous organic solvent (Japanese Patent Laid-Open No.188,601/1996) are known.

With an increasing awareness of environmental problems in recent years,starch esters produced by the, methods described above have been used invarious biodegradable plastic materials. However, these materials,whether used alone for forming molded articles or films or incombination with various synthetic resins, require a general-purposeplasticizer (phthalate type or fatty ester type) in order to achieveworkability (for example, injection workability, extrusion workability,stretchability, etc.) at the same levels as ordinary thermoplasticplastics (thermoplastic resin).

Even if produced using the plasticizer, products such asinjection-molded articles hardly achieve impact strength at the samelevels as with impact strength polystyrene (high impact polystyrene). Ithas also been difficult to achieve molded articles having an impactresistance of 1.8 kgf·cm/cm (17.64 J/m) or more in terms of Izod impactstrength (ASTM D256: −23° C.).

Further, products such as inflation films have hardly achievedstretchability (tensile elongation) as good as that of polyethylene.

In particular, these tendencies become significant as the ratio of thestarch ester in the plastic composition (plastic material) to be moldedis increased.

Even if a biodegradable resin (biodegradable polymer) other than thestarch ester is mixed in an attempt to improve the impact strength ortensile elongation of the starch ester, the desired improvement effectscannot be attained unless the content of the biodegradable resin is madeto be higher than the content of the starch ester. As a result, suchproducts cannot truly be said to be biodegradable plastics that arebased on a starch ester.

Further, the phthalate or fatty ester type plasticizer described aboveis suspected of being a physiologically disturbing substance, whichadversely affects vegetables, foods, and the growth of animals.Accordingly, one should avoid adding the plasticizer described above tobiodegradable plastics that are to be disposed of in landfills, etc.

In view of the foregoing, an object of the present invention is toprovide a starch ester which can be used as a thermoplastic materialcapable of being thermo-plasticized in the absence of a plasticizer orby using a small amount of a plasticizer.

Another object of the present invention is to provide a starch esterfrom which a thermoplastic plastic material having superior impactstrength and tensile elongation can be easily prepared.

DISCLOSURE OF INVENTION

The present inventors made extensive study regarding the development ofsafe biodegradable plastics in the absence of a plasticizer or by usinga small amount of a plasticizer, by use of starch which is an abundantraw material produced every year. The result of these studies is thenovel starch ester having the constitution described below.

The present invention relates to a starch ester wherein reactivehydroxyl groups in the same starch molecule have been replaced by a C₂₋₄acyl group (referred to hereinafter as “short-chain acyl group”) and aC₆₋₁₈ acyl group (referred to hereinafter as “long-chain acyl group”),and the extent of substitution by the short- and long-chain acyl groupsare regulated so as to make the starch ester thermo-plasticized andmoldable even in the absence of a plasticizer.

From the viewpoint of workability, the starch ester as used herein ispreferably one having a glass transition point by differential thermalanalysis (JIS K 7121: referred to hereinafter as “glass transitionpoint”) of 140° C. or less, preferably 130° C. or less. The lower limitof the glass transition point shall be usually 80° C., preferably 100°C.

To easily attain each characteristic, a starch ester having theworkability or showing the glass transition point as described above ispreferably one wherein the degree of substitution by the long-chain acylgroup is from 0.06 to 2.0, the degree of substitution by the short-chainacyl group is from 0.9 to 2.7, and the degree of substitution by thetotal acyl groups is from 1.5 to 2.95, more preferably one wherein thedegree of substitution by the long-chain acyl group is from 0.1 to 1.6,the degree of substitution by the short-chain acyl group is from 1.2 to2.1, and the degree of substitution by the total acyl groups is from 1.7to 2.9.

The starch ester of the present invention can also be used in a starchester-based polymer alloy by incorporating the starch ester with abiodegradable resin. Polycaprolactone, polylactic acid or celluloseacetate can be used particularly preferably as the biodegradable resin.

Further, the starch ester of the present invention can be formed into amolded processed article which has been molded and processed as a wholeor partially with said starch ester or a polymer alloy having saidstarch ester incorporated with a biodegradable resin.

The molded processed article can be formed into an injection-moldedarticle showing a degree of water absorption (after immersion in tapwater at 23° C. for 24 hours) of 0.5% or less and an Izod impactstrength of 1.8 kgf·cm/cm, or into a film having a film thickness of 100μm or less and a tensile elongation (JIS K 6301) of 200% or more.

Further, the starch ester of the present invention can be formed into aplastic processed article which has been molded and processed, ormodified, as a whole or partially with a plastic composition comprisingan organic or inorganic reinforcing filler added to said starch ester orto a polymer alloy which is an admixture of the starch ester and abiodegradable resin.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the means of the present invention is described in detail.The blend unit is expressed on a weight basis unless otherwisespecified. In the following description, Cn in round brackets after eachcompound indicates that the number of carbons in acyl groups in thecompound is n.

As used herein, the degree of substitution (DS) is the average number ofreactive hydroxyl groups (that is, 3 hydroxyl groups at the 2-, 3- and6- or 4-positions) replaced by substituent groups per glucose residue ina starch derivative, and when DS is 3, the degree of masking(substitution percent) of the reactive hydroxyl groups is 100%.

As a result of intensive study for solving the problem described above,the present inventors found that it is essential to conferthermoplasticity on starch itself in a practical temperature range inorder to solve the problem, and it is important therefor to bindlong-chain hydrocarbon-containing groups such as long-chain alkylgroups, cycloalkyl groups, alkylene groups and aryl groups, along withshort-chain hydrocarbon-containing groups such as short-chain alkylgroups, cycloalkyl groupa, alkylene groups and aryl groups, to the samestarch molecule. By so doing, the present invention arrived at the novelstarch ester with the constitution described below.

Said starch ester is conceptually shown in the structural formula:

wherein. R¹ is a C₂₋₄ short-chain acyl group, and R² is a C₆₋₁₈long-chain acyl group.

Although the process for producing said starch ester is not particularlylimited, the starch ester can be easily produced by a processconstituted as follows (see Japanese Patent Laid-Open No. 188,601/1996(Japanese Patent No. 2,579,843)): “A process for producing a starchester by using a vinyl ester as an esterification reagent wherein avinyl ester having a C₂₋₁₈ ester, group is reacted with starch in anon-aqueous organic solvent using an esterification catalyst.”

That is, the biodegradable starch ester of the present invention can beeasily synthesized through acylation (esterification) in a non-aqueousorganic solvent by replacing reactive hydroxide groups in the samestarch molecule by long-chain acyl groups derived from vinyl compounds,acid anhydrides acid halides, alkyl ketene dimers or lactones, alongwith short-chain acyl groups from the same compounds.

By such means, the present inventors found for the first time that:

1) By these reactive groups, it is possible to synthesize starch estersexhibiting thermoplasticity during heating in the absence of aplasticizer or by using a small amount of a plasticizer;

2) These starch esters show a significantly higher miscibility withbiodegradable resins other than said starch esters than that of existinghighly modified starch esters (prepared by the known method describedabove); and

3) Molded processed articles formed as a whole or partially from aplastic composition based on said starch esters have impact resistancesimilar to that of impact-resistant (high impact) polystyrene.

As the starting starch for the starch ester of the present invention,(1) unmodified starch from on the ground (soil), such as corn starch,high amylose starch, wheat starch and rice starch, (2) unmodified starchin the ground, such as potato starch and tapioca starch, and (3) starchesters prepared by subjecting the above-described starches to low-degreeesterification, etherification, oxidation, acid treatment, or conversioninto dextrin; these starches can be used alone or in a combinationthereof.

The acylation (esterification) reagent used for introducing C₆₋₁₈long-chain acyl groups onto reactive hydroxyl groups by substitutionreaction includes one or more members selected from alkyl ketene dimers,cyclic esters (caprolactones), acid anhydrides, acid halides and vinylcompounds having esterification (acylation) reactive sites having C₅₋₁₇long-chain hydrocarbon groups bound to carbonyl groups (number ofcarbons in one molecule of the reagent: 6 to 18).

The long-chain hydrocarbon groups described above include an alkylgroup, a cycloalkyl group, an alkylene group and an aryl group as wellas groups derived therefrom. The derived groups include an aryl alkylgroup (aralkyl), alkyl aryl group (alkaryl), and alkoxy alkyl group. Thelong-chain hydrocarbon groups also include active hydrogen groups suchas a hydroxy alkyl group and an aminoalkyl group, insofar as the effectof the present invention is not adversely affected.

Among these compounds, esterification reagents having C₈₋₁₄esterification reaction sites are preferable for reaction efficiency andhandling.

The alkyl ketene dimers are constituted of a combination of variousalkyl groups, as represented by the formula:

wherein R is a C₅₋₁₇ alkyl group, an alkylene group, an aryl group, or agroup derived therefrom.

As the cyclic esters (caprolactones), ε-caprolactone (C6),γ-caprylolactone (C8), γ-laurolactone (C12) and γ-stearolactone (C18),as well as large cyclic lactones represented by the formula (CH₂)_(n)COOwherein n is an integer from 5to 17; these can be used singly or incombination thereof.

As the acid anhydrides and acid halides, anhydrides and halides ofcaprylic acid (C8), lauric acid (C12), pal mitic acid (C16), stearicacid (C18), oleic acid (C18), etc., can be used.

As the vinyl compounds, it is possible to use saturated or unsaturatedvinyl aliphatic carboxylates such as vinyl caprylate (C8), vinyl laurate(C12), vinyl palmitate (C16), vinyl stearate (C18) and vinyl oleate(C18), and branched saturated vinyl aliphatic carboxylates representedby the following structural formula:

wherein all R¹, R² and R³ are alkyl groups, and the number of carbons inthese groups in total is from 4 to 16.

The non-aqueous polar organic solvent is one capable of dissolving thestarting starch, and specifically, dimethyl sulfoxide (DMSO), dimethylformamide (DMF), pyridine, etc., can be used alone or in a combinationthereof, or these can be used as a mixture with another organic solvent.

The esterification (acylation) catalyst used includes one or moremembers selected from the following respective groups: (1) hydroxides,mineral acid salts, carbonates, organic compounds or alkali metalalkoxides of metals up to the 5th period in the Periodic Table, (2)organic-interlayer transfer catalysts, and (3) amino compounds. Amongthese, (1) is desirable from the viewpoint of reaction efficiency andcatalyst costs.

Examples of the catalysts are as follows:

(1) Alkali metal hydroxides such as sodium hydroxide, potassiumhydroxide and lithium hydroxide; alkali metal organic acid salts such assodium acetate and sodium p-toluene sulfonate; alkaline earth metalhydroxides such as barium hydroxide and calcium hydroxide, alkalineearth metal organic acid salts such as calcium acetate, calciumpropionate and barium p-toluene sulfonate; inorganic acid salts such assodium phosphate, calcium phosphate, sodium hydrogen sulfite, sodiumcarbonate, sodium hydrogen carbonate, potassium carbonate, potassiumhydrogen carbonate, potassium sulfate, sodium aluminate and potassiumzincate; and amphoteric metal hydroxides such as aluminum hydroxide andzinc hydroxide; and

(2) Amino compounds such as dimethyl amino pyridine and diethyl aminoacetic acid,

as well as quaternary ammonium compounds such as N-trimethyl-N-propylammonium chloride and N-tetraethyl ammonium chloride. The timing andmethod of using these catalysts are not particularly limited.

The acylation (esterification) reagent used for introducing C₂₋₄short-chain acyl groups onto reactive hydroxyl groups by substitutionreaction includes one or more members selected from alkyl ketene dimers,cyclic esters (caprolactones), acid anhydrides, acid halides and vinylcompounds having esterification (acylation) reaction sites having C₁₋₃short-chain hydrocarbon groups bound to carbonyl groups (that is, thenumber of carbons in each reagent molecule is from 2 to 4).

Specifically, the following compounds can be mentioned. Among thesecompounds, those acylation reagents having C₂₋₃ esterification reactionsites are preferable for reaction efficiency, microbial degradation andhandling.

As the cyclic esters (caprolactones), γ-butyrolactone (C3) andβ-propiolactone (C3) can be used singly or in combination thereof.

As the acid anhydrides and acid halides, anhydrides and halides ofacetic acid (C2), propionic acid (C3), butanoic acid (C4), etc., can beused singly or in combination thereof.

As the vinyl compounds, vinyl acetate (C2), vinyl propionate (C3), vinylbutanoate (C4), vinyl acrylate (C3), vinyl isocrotonate (C4), etc., canbe used.

Although the reaction temperature condition in the present invention isnot particularly limited, the reaction temperature shall be usually 30°C. to 200° C., preferably 40° C. to 150° C. For almost all compounds, itwill not be necessary to change these reaction temperatures.

With respect to the degree of substitution (DS) on the starch ester, thecompatibility of the starch ester with a biodegradable resin as anobject of the present invention is affected by, the length of thelong-chain acyl group. However, with the long-chain acyl group havingthe maximum number of carbons given, it is difficult to confer thedesired characteristics on starch insofar as the degree of substitution(DS) by said acyl group is 0.05 or less (or the degree of masking ofreactive hydroxyl groups is 2% or less). As the acyl group having themaximum number of carbons, the acyl group containing 19 or more carbonatoms is not practical because the reaction efficiency is extremelylowered thereby.

Usually, the DS by the long-chain acyl group is from 0.06 to 2.0 (degreeof masking: 2% to 67%), the DS by the short-chain acyl group is from 0.9to 2.7 (degree of masking: 30% to 90%), and the DS by the total acylgroups is from 1.5 to 2.95 (degree of masking: 50% to 98%).

Between the starch ester wherein the degree of substitution by thelong-chain acyl group is minimal and the degree of substitution by theshort-chain acyl group is maximal, and the starch ester wherein thedegree of substitution by the long-chain acyl group is maximal and thedegree of substitution by the short-chain acyl group is minimal, thereis no extreme difference in the compatibility thereof with abiodegradable resin nor in the mechanical.physical properties thereof.To achieve the same level of thermoplasticity in the absence of aplasticizer, the degree of substitution by the long-chain acyl group maybe decreased as the number of carbons in said acyl group is increased.

Accordingly, the above-described numerical range has no particularcritical importance, and the present invention can be carried out evenin the vicinity of the above-described range.

Preferably, the DS by the long-chain acyl group is from 0.1 to 1.6(degree of masking: 3% to 53%), the DS by the short-chain acyl group isfrom 1.2 to 2.1 (degree of masking: 40% to 70%), and the DS by the totalacyl groups is from 2.0 to 2.9 (degree of masking: 67% to 97%).

The reason that the number of carbons in the short-chain acyl groupshall be 4 or less is based on the experimental result that in thepresent invention, there is no difference in reaction efficiency amongC₂₋₄ short-chain acyl groups.

With respect to the glass transition point (JIS K 7121) of the starch,the miscibility of the starch with the biodegradable resin becomesgradually poor as the transition point (transition temperature) isincreased. Usually, the glass transition point shall be 140° C. or less,preferably 80° C. to 130° C. This is because if the glass transitionpoint is higher than 140° C., miscibility becomes poor in the absence ofa plasticizer. If a plasticizer is used, the starch ester is renderedmiscible even at a temperature of higher than 140° C. and in a smalleramount of the plasticizer than is conventional.

Hereinafter, the biodegradable polymer (biodegradable resin)incorporated with the starch ester of the present invention to form apolymer alloy is described.

In the present invention, the term “incorporated” means that two or morematerials are admixed with “compatibility”, and the term “compatibility”refers to the state of two or more materials in which they are uniformlyand mutually dispersed, including not only the state attained by mixingtwo or more materials having mutual miscibility, but also the statewhere two or more materials, although being mutually “immiscible”, areuniformly dispersed.

As can also be easily judged from the above-described glass transitiontemperature (glass transition point), the starch ester of the presentinvention can be thermoplasticized without using an oily plasticizer.Further, the starch ester of the present inventions does not require anyplasticizer for blending thereof with the existent biodegradable resin,and the compatibility thereof is significantly improved as compared withthat of starch esters produced in the prior art, such ashigh-substituted acetylated starch (acetate starch).

As the biodegradable polymer described above, the following polymers ofa natural type (mainly cellulose type) or of a synthetic type(polymerized type) can be preferably used.

That is, the polymers of cellulose type include cellulose acetate,hydroxyethyl cellulose, propyl cellulose, hydroxybutyl cellulose, etc.

The polymers of polymerized type include:

(1) biodegradable polyesters or polyamides such as polycaprolactone(PCL), polylactic acid (PLA), polyadipate, polyhydroxy butyrate(polyhydroxy alkanoates), polyhydroxy butyrate valerate (PHB/V) andsuccinic acid-1,4-butanediol polymers;

(2) polyalkylene oxides such as polyethylene oxide and polypropyleneoxide; and

(3) vinyl polymers such as polyvinyl alcohol, modified polyvinylalcohol, polyacrylamide-based resin, polycarbonate-based resin,polyurethane-based resin, polyvinyl acetate, polyvinyl carbazole,polyacrylate, and ethylene-vinyl acetate copolymers.

When the starch ester of the present invention or the polymer alloydescribed above is used as a base polymer to prepare a plastic material(starch ester-based composition), the following various fillers can beused as fillers that are used together with other auxiliary materials.

The form of the fillers can be selected arbitrarily as necessary frompowder, granules, plates, cylinders, fibers and needles.

The inorganic fillers include talc, titanium oxide, clay, chalk,limestone, calcium carbonate, mica, glass, silica and various silicasalts, diatomaceous earth, wall austenite, various magnesium salts,various manganese salts etc.

The organic fillers include starch and starch derivatives, cellulose andderivatives thereof, wood powder, pulp, pecan fibers, cotton powder,corn husk, cotton linter, wood fibers, bagasse, etc.

The synthetic fillers include glass fibers, urea polymers, ceramics,etc.

(1) Hereinafter, the Examples, Comparative Examples and ApplicationExamples conducted for confirming the effect of the present inventionare described.

EXAMPLE 1

25 g high amylbse corn starch was suspended in 200 g dimethyl sulfoxide(DMSO), then heated to 90° C. under stirring, and kept at thistemperature and gelatinized. 20 g of sodium bicarbonate was added as acatalyst to this solution, and while the temperature was kept at 90° C.,17 g of vinyl laurate (C12) was added thereto and reacted at the sametemperature for 1 hour. Then, 37 g of vinyl acetate (C2) was addedthereto, and reacted in the same manner as above at 80° C. for 1 hour.Thereafter, the reaction solution was poured into tap water, thenstirred at high speed and ground, filtered, dehydrated and dried toprepare the starch ester of Example 1.

EXAMPLE 2

The starch ester of Example 2 was prepared in the same manner as inExample 1 except that acid-treated regular corn starch was used in placeof high amylose corn starch, and 14 g vinyl stearate (C18) was used inplace of vinyl laurate.

EXAMPLE 3

The starch ester of Example 3 was prepared in the same manner as inExample 1 except that 16 g chlorinated stearic acid (C18) was used inplace of vinyl laurate.

EXAMPLE 4

100 g commercial corn starch with a water content reduced to 1% or lessby preliminary drying, and 800 g DMSO, were introduced into a 2 Lseparable flask equipped with a stirrer, then heated at 90° C. andgelatinized by keeping it at this temperature for 20 minutes. After asolution of 5 g t-butyl bromide and 532 g hexadecyl ketene dimer (C17)was added dropwise thereto, the mixture was reacted at 90° C. for 5hours in a system under reduced pressure, during which the DMSO wasrefluxed. Thereafter, the reaction system was returned to theatmospheric pressure, and a solution of 126 g acetic anhydride and 103.8g sodium bicarbonate was added dropwise thereto, and the mixture wasreacted for 1 hour at the reflux temperature thereof. After theunreacted materials and byproducts were allowed to flow out, the productwas recovered under vigorous stirring in water, and then washedrepeatedly 5 times with 5 L water to prepare the starch ester of Example4.

EXAMPLE 5

The mixed ester of Example 5 was prepared in the same manner as inExample 1 except that 18.5 g of vinyl 2,2-dimethyltridecanoate (C15) wasused in place of vinyl laurate.

EXAMPLE 6

The starch ester of Example 6 was prepared in the same manner as inExample 1 except that 27 g vinyl hexanoate was used in place of vinyllaurate.

COMPARATIVE EXAMPLE 1

The starch ester (starch acetate) of Comparative Example 1 was preparedin the same manner as in Example 1 except that 39.9 g vinyl acetate (C2)only was used.

COMPARATIVE EXAMPLE 2

25 g of high-amylose corn starch was suspended in 200 g DMSO and heatedto 80° C. and gelatinized by keeping it at this temperature for 20minutes. After 39 g of sodium bicarbonate was added for neutralizationof an acid produced as a byproduct in this solution, the solution wascooled to a reaction temperature of 20° C., and 48 g acetic anhydridewas added thereto at a reaction temperature kept at 20° C. to 25° C. soas to prevent hydrolysis of the starch. After acetic anhydride wasadded, the mixture was reacted at the same temperature for 1 hour.Thereafter, the starch ester of Comparative Example 2 was prepared inthe same manner as in Example 1.

COMPARATIVE EXAMPLE 3

46 g of high-amylose starch was introduced into a 1-L four-necked flaskequipped with a reflux condenser, a dropping funnel and a thermometer,and 150 ml acetic anhydride was added thereto under stirring. Then, themixture was heated until a certain reflux occurred. The boilingtemperature was about 125° C. After 1 to 2 hours, the viscosityincreased, and after 3 to 4 hours, a viscous brownish transparentmixture was generated. After around 5 hours which was a necessaryreaction time, 5 to 10 ml acetic acid was fractionated at 118° C., andthen 20 ml ethanol was added dropwise to the reaction solution. Thereaction solution.was further stirred for 30 minutes under slightlysuppressed heating. Then, the solvent mixture consisting of ethylacetate and acetic acid, generated by the reaction of the ethanol withthe acetic anhydride, was fractionated at 102° C. to 105° C. Then,heating was stopped, and the mixture was cooled for 0.5 to 1 hour.Subsequently, 20 ml ethanol was added again dropwise thereto.Thereafter, the product was gradually precipitated with about 200 mlmethanol. The product was washed several times with alcohol, separatedunder suction and air-dried to prepare the starch ester of ComparativeExample 3.

(2) The respective starch esters prepared in the Examples andComparative Examples above were tested in regards to each of thefollowing items.

Test 1

The degree of substitution (DS) by long- and short-chain acyl groups,and the glass transition point, were measured using the followingrespective methods;

(1) Degree of Substitution by Long- and Short-chain Acyl Groups

Measured in accordance with the saponification method (Genung & Mallet,1941) (see “Starch/Related Glucide Experimental Method”, p. 291,published on October 10, 1986, by K. K. Gakkai Shuppan Center).

The saponified product (alkali-hydrolyzed product) in a liquid phase,obtained by the above method, was separated and quantified by liquidchromatography for the ratios of long- and short-chain aliphatic acidssimultaneously, to determine the degrees of substitution by long- andshort-chain acyl groups.

(2) Glass Transition Point

Determined according to JIS K7121 by using the “Differential ScanningCalorimeter DSC-50” (Shimadzu Corporation).

The results are shown in Table 1. As can be seen from the table, theglass transition point is significantly lower in the presence of thelong-chain acyl group than in the absence of the long-chain acyl group,even at almost the same degree of substitution of hydroxyl groups. Thissuggests that the starch ester can be thermo-plasticized in the absenceof a plasticizer.

TABLE 1 Glass DS by DS by trans. long chain short chain Total DS pointExample 1 0.5 (C12) 1.95 (C2) 2.45 110° C. Example 2 0.3 (C18) 1.95 (C2)2.25 120° C. Example 3 0.3 (C18) 1.93 (C2) 2.23 117° C. Example 4 0.23(C17) 1.89 (C2) 2.12 103° C. Example 5 0.45 (C15) 1.88 (C2) 2.33 115° C.Example 6 1.43 (C6) 1.35 (C2) 2.78 105° C. Comparative Example 1 0 2.45(C2) 2.45 165° C. Comparative Example 2 0 2.40 (C2) 2.40 170° C.Comparative Example 3 0 2.20 (C2) 2.20 167° C.

Test 2

30 parts of the indicated biodegradable polymer was added to and mixedwith 100 parts of each starch ester and mixed (mixing means:plastomill). The resulting compound was extruded into a 40 μm film(width, 120 mm) through a twin-screw extruder (L/D=32), and on the basisof the transparency of the film, the compatibility of each starch esterwith the biodegradable polymer was judged.

In the Comparative Examples, 100 parts of each starch ester wasplasticized by adding 40 parts of triacetin (glycerol triacetate) as theplasticizer, which was then mixed with the biodegradable polymer andthermo-plasticized.

The extrusion conditions were as follows: the plasticization temperaturewas 170° C., the T-die temperature was 170° C., the extrusion rate was10 m/min., and the output was 3 kg/min.

The results are shown in Table 2. In the Examples where the long-chaingroup is introduced, all the products are judged to be transparent andhighly compatible. On the other hand, it is judged that in theComparative Examples where the short-chain group only is used, all theproducts are opaque, and even if the plasticizer is used, theircompatibility is inferior.

TABLE 2 PCL PLA Acetate Cellulose Example 1 transparent transparenttransparent Example 2 transparent transparent transparent Example 3transparent transparent transparent Example 4 transparent transparenttransparent Example 5 transparent transparent transparent Example 6transparent transparent transparent Comparative Example 1 opaque opaqueopaque Comparative Example 2 opaque opaque opaque Comparative Example 3opaque opaque opaque

Test 3

15 parts of PCL were mixed with 100 parts of each starch ester (or inthe Comparative Examples, with 100 parts of each starch ester afterbeing thermo-plasticized by adding 20 parts of triacetin) and thenthermo-plasticized to prepare a polymer alloy. The polymer alloy wasused to prepare a test specimen, and the degree of water absorption andIzod impact strength of each test specimen were determined using thefollowing methods.

Degree of Water Absorption

An injection-molded plastic disk (diameter 50 mm×thickness 3 mm)prepared from each specimen was immersed in tap water at 23° C. for 24hours, and then examined for the amount of water absorbed therein.

Izod Impact Strength

Determined at an atmospheric temperature of −23° C. in accordance withASTM D256.

The test results are shown in Table 3. As can be seen from the results,in the Examples where both the short- and long-chain groups areintroduced, the degrees of water absorption are significantly lower than(nearly hundred times as low as) that of the Comparative Examples wherethe short-chain group only is used, and the Izod impact strength is alsosignificantly higher than in the Comparative Examples.

TABLE 3 Izod impact Degree of water absorption strength (%) (kgf ·cm/cm) Example 1 0.1 4.5 Example 2 0.1 5.1 Example 3 0.2 4.8 Example 40.1 4.5 Example 5 0.12 4.7 Example 6 0.1 4.1 Comparative Example 1 10.51.0 Comparative Example 2 8.0 0.6 Comparative Example 3 11.0 0.7

Test 4

40 parts of PCL were mixed with 100 parts of each starch ester (or inthe Comparative Examples, with 100 parts of each starch ester afterbeing thermo-plasticized by adding 40 parts of triacetin) and thenthermo-plasticized to prepare a polymer alloy. The polymer alloy wasused to prepare a thin film by an inflation processing unit (blowingbore diameter, 100 mm; cylinder diameter, 150 mm), and thecharacteristics thereof in the following items were observed ormeasured.

Inflation State: Visual Observations.

Film thickness: Five sites of each film were measured by a micrometerfor thickness, to determine the average thickness.

Film tensile elongation (E_(B)): Determined according to JIS K 6301.

The test results are shown in Table 4. High miscibility among resins isrequired of resins, in particular of blended resins, in order to preparethin films by the inflation method. Mutually immiscible resins, even ifthey seem to be uniformly blended at a glance, are broken in forming afilm due to insufficient melt elongation and tensile elongation. Apolymer characterized by being capable of forming a thin film withoutbreakage as described above is considered as worthy of a status that isdifferent from polymers not having such characteristics.

TABLE 4 State of thickness of thin tensile elongation inflation film offilm Example 1 good 40 μm 560% Example 2 good 20 μm 600% Example 3 good40 μm 450% Example 4 good 40 μm 500% Example 5 good 25 μm 380% Example 6good 25 μm 350% Comparative Example 1 broken not formable — ComparativeExample 2 broken not formable — Comparative Example 3 broken notformable —

Industrial Applicability

As substantiated in the Examples and Comparative Examples describedabove, the starch ester of the present invention having long- andshort-chain substituent groups consisting of alkyl, alkylene or arylbound via an ester-type linkage to the same molecule can bethermo-plasticized, molded and processed without requiring anyester-based plasticizer which is needed for plasticizing conventionalstarch esters, other processed starch or unmodified starch.

Further, the starch ester of the present invention surprisinglydemonstrates very high compatibility with biodegradable polymers such asconventional synthetic and fermented polyesters, and thus biodegradablepolymer alloys meeting the performance required of products can beproduced.

That is, by use of the starch ester of the present invention, athermoplastic plastic material (starch ester-based composition)excellent in impact strength and tensile elongation can be easilyprepared.

What is claimed is:
 1. A polymer alloy which comprises a starch esterincorporated with a biodegradable resin, wherein hydrogens in reactivehydroxyl groups in the same starch molecule have been replaced by C₂₋₄acyl groups (referred to hereinafter as “short-chain acyl group”) andC₆₋₁₈ acyl groups (referred to hereinafter as “long-chain acyl group”),the degree of substitution by the short- and long-chain acyl groups areregulated so as to make the polymer alloy thermo-plasticized andmoldable even in the absence of a plasticizer; and wherein the glasstransition point of the starch ester by differential thermal analysis(JIS K 7121: referred to hereinafter as the “glass transition point”) is130° C. or less.
 2. The polymer alloy according to claim 1, wherein saidbiodegradable resin is any one selected from the group consisting ofpolycaprolactone, polylactic acid, cellulose acetate and mixturesthereof.
 3. A plastic processed article which has been molded andprocessed or modified as a whole or partially with the polymer alloy ofclaim
 1. 4. The plastic processed article according to claim 3, which isan injection-molded article showing a degree of water absorption (afterimmersion in tap water at 23° C. for 24 hours) of 0.5% or less and anIzod impact strength of 1.8 kgf·cm/cm or more.
 5. The plastic processedarticle according to claim 3, which is a film having a film thickness of100 μm or less and a tensile elongation (JIS K 6301) of 200% or more. 6.A plastic processed article which has been molded and processed as awhole or partially with a plastic composition comprising an organic orinorganic reinforcing filler added to the polymer alloy of claim
 1. 7. Aplastic processed article which has been molded and processed ormodified as a whole or partially with a polymer alloy, wherein thepolymer alloy comprises a starch ester incorporated with a biodegradableresin, wherein hydrogens in reactive hydroxyl groups in the same starchmolecule have been replaced by C₂₋₄ acyl groups (referred to hereinafteras “short-chain acyl group”) and C₆₋₁₈ acyl groups (referred tohereinafter as “long-chain acyl group”), the degree of substitution bythe short- and long-chain acyl groups are regulated so as to make thepolymer alloy thermo-plasticized and moldable even in the absence of aplasticizer, and wherein the plastic processed article is aninjection-molded article showing a degree of water absorption (afterimmersion in tap water at 23° C. for 24 hours) of 0.5% or less and anIzod impact strength of 1.8 kgf·cm/cm or more.